The light emitting device is, for example, an inorganic LED, an organic LED, a laser diode, an inorganic thick film electroluminescence sheet, or an inorganic thin film electroluminescence unit.
In particular, the LED is outstanding for the characteristics of a long life, the absence of necessity of a wide space, the strength against the impact, and further for the light emission in a narrow spectrum band.
The inherent light emission from an active semiconductor material of LED does not offer sufficiently a number of emission light colors, in particular, a number of emission light colors with a wide spectrum band. This is true of, in particular, the case that white color light emission is targeted.
From the state of the art, even an emission light color unavailable originally by semiconductors can be obtained by a color conversion technique.
The color conversion technique is essentially based on the following principle: that is, at least one phosphor is disposed on an LED die; the phosphor absorbs the light emission from the die; and then it emits photoluminescence light in another light emission color.
To compose the phosphor, basically, an organic material is available and an inorganic material is also available. The essential advantage of inorganic pigment is that it has a higher environment resistance than an organic based phosphor. In consideration of the color stability based on the long life of inorganic LED, the inorganic material is more advantageous.
In consideration of the processing easiness, it is apparently advantageous to use an inorganic fluorescent pigment instead of an organic fluorescent coat based phosphor that requires an excessively long growth period to obtain a necessary film thickness. The pigment is added into the matrix, and then placed on the LED die.
From the reason that the number of inorganic materials satisfying the above-mentioned demands is small, YAG group materials are, at present, used as the pigment for the color conversion in most cases. However, the YAG group materials have a disadvantage that they show a high efficiency only when the light emission maximum value is less than 560 nm. Because of this, when using a YAG pigment in combination with a blue diode (450 nm and 490 nm), only a white emission light color with a cold feeling can be realized. Especially, in the field of lighting, there is a higher demand concerning the color temperature and the color reproduction. This demand cannot be satisfied by white LED's available now.
The International publication No. WO 00/33389 discloses that Ba2SiO4:Eu2+ can be used as the phosphor to get light close to white in using a blue LED. The emitted light of Ba2SiO4:Eu2+ has a relatively short wavelength of 505 nm, and therefore, the light is remarkably in cold color.
S. H. M. Poort et al., “Optical properties of Eu2+-activated” page 297 reports the properties of Ba2SiO4 and a phosphate such as KbaPO4 and KSrPO4 that are activated by Eu2+. In this report, it is confirmed that the light emission of Ba2SiO4 is at 505 nm. Furthermore, it is reported that the light emission of the two phosphates are essentially at a further shorter wavelength (420 nm to 430 nm)